In recent years, there has been a concerted effort on the part of the managements of electrical generating stations to upgrade the steam turbines in many older steam turbine-generator units across the United States and elsewhere. There are two directions, generally, to pursue: (1) Upgrading the steam path components such as the vanes (buckets, blades) on the rotors and/or vanes (partitions) in the stationary components (diaphragms, blade rings) while leaving the existing rotors and shells in place, and upgrading adjacent components such as bearings, and (2) Replacing the existing rotors and shells with upgraded rotors and shells, and replacing adjacent components such as bearings with upgraded bearings. In almost all situations, these upgraded designs result from the use and application of advanced technology that is dependent upon the use of digital computers that were not available when the original steam turbines were designed and built.
A major objective is to maintain tight seal clearances in the steam path in order to improve the steam path efficiency. This requires low amplitude rotor vibrations, which can only result from optimized rotor-bearing dynamics for the turbine rotor and bearings.
Since the beginning of the turbine-generator industry, there has been significant pressure to improve the rotor vibration measuring instrumentation and to improve the calculation capabilities for predicting the vibratory characteristics of the rotors in rotor-bearing systems. In the 1940s and 1950s, the instrumentation was rudimentary at best. For rotor-dynamics and film bearing design, the fundamental equations were known, but only the simplest of equations could be solved with the “closed form” analytical methods. With the introduction of digital computers, rotor-dynamics and film bearing designs for systems other than the simplest could be calculated. As the digital computer technology advanced, larger and more complex rotor-bearing-foundation models could be modeled. The results of computer simulations demonstrate the importance of controlling the stiffness of foundations for film bearings for various steam turbine rotors. At the same time, the introduction of solid-state electronics, and then later, the introduction of computer-based instrumentation and diagnostics provided great advances in the ability to monitor, diagnose, and record vibrations of turbine rotors under a wide range of operating conditions.
The results of almost all of the rotor-dynamic studies of a class of large steam turbine rotors known as “high-pressure steam turbine” rotors clearly show that very stiff pedestals for the oil-film bearings that support these rotors is highly advantageous for minimizing the vibratory amplitudes of the rotors relative to the turbine shells. The vibratory instrumentation and equipment for monitoring, diagnosing, and recording the vibrations of these rotors confirms these results. Because there is great value in optimizing the efficiency, availability, and reliability of these steam turbines and their associated generators, there is considerable justification to improve the design of components when and where possible.
This invention has particular application to the replacement of flexible bearing supports of a type used in Westinghouse Electric Company steam turbines from the late 1940s to the mid 1960s, but its utility is not confined thereto.
These original bearing and support arrangements were simple, comprising a yoke, a bearing mounted in a yoke, a bearing cap to retain the bearing in the yoke, and two flexible I-beams to support a yoke. In this type of steam turbine, the bearing design is typically steel backed, Babbitt lined, oil film lubricated, and not of rolling element design. To change the alignment of the rotors it is necessary to change the position of the bearings supporting the rotors. Shims are used between the yoke and pedestals to adjust the vertical height of the yokes and bearings, and jack screws or shims are used to adjust the horizontal positions of the yokes, and hence, of the bearings. This design is discussed in considerable detail in my U.S. Pat. No. 6,712,516.
In the original arrangement, when a turbine rotor with a total weight on the order of 30,000 lbs (133,440 Newtons) is placed in two bearings of this design, the vertical displacement downward of each bearing is approximately 0.003 to 0.005 inches (0.229-0.635 millimeters). This provides a vertical stiffness of the bearing and flexible support on the order of between 5 million lbs/inch (15,000 lbs/0.003 inches) and 3 million lbs/inch. On the other hand, a vertical stiffness of the bearing support of at least 15 million lbs/inch is required to obtain superior rotor-dynamic characteristics, and even higher vertical stiffnesses of the support structures for the film bearings are preferred.
One object of this invention is to provide a method to replace an existing yoke and mating flexible supports having low stiffness, on the order of 5 million lbs/in with a substantially stiffer pedestal having a desired stiffness of at least about 15 million lbs/in, hereinafter stiff plate.
Another object of this invention is to provide improved means for aligning the bearing and rotor.
Other objects will occur to those skilled in the art in light of the following description and accompanying drawings.
It should be noted that from the perspective of improved rotor-dynamics, another class of bearing support pedestals exists that is superior to the original spring plate pedestals that are being replaced by the stiff pedestal of this invention. This other class of bearing support pedestals uses heavy steel plates welded into, and therefore, integral with, the outer pedestal housings. It has been used by all manufacturers of turbines for at least as long as the spring plate pedestal has been used, and is satisfactory for providing adequate stiffness for the oil film bearings used. Upgraded pedestals using the stiff pedestal design and methods of this invention are comparable in stiffness to this other class of bearing pedestals that were welded into and integral with the outer pedestal housings.